6.11.2 Object Properties

It’s often useful to associate a piece of additional information with a
Scheme object even though that object does not have a dedicated slot
available in which the additional information could be stored. Object
properties allow you to do just that.

Guile’s representation of an object property is a procedure-with-setter
(see Procedures with Setters) that can be used with the generalized
form of set! (REFFIXME) to set and retrieve that property for any
Scheme object. So, setting a property looks like this:

To create an object property in the first place, use the
make-object-property procedure:

(define my-property (make-object-property))

Scheme Procedure: make-object-property

Create and return an object property. An object property is a
procedure-with-setter that can be called in two ways. (set!
(propertyobj) val) sets obj’s property
to val. (propertyobj) returns the current
setting of obj’s property.

A single object property created by make-object-property can
associate distinct property values with all Scheme values that are
distinguishable by eq? (including, for example, integers).

Internally, object properties are implemented using a weak key hash
table. This means that, as long as a Scheme value with property values
is protected from garbage collection, its property values are also
protected. When the Scheme value is collected, its entry in the
property table is removed and so the (ex-) property values are no longer
protected by the table.

Guile also implements a more traditional Lispy interface to properties,
in which each object has an list of key-value pairs associated with it.
Properties in that list are keyed by symbols. This is a legacy
interface; you should use weak hash tables or object properties instead.